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Researchers say millions of "missing girls" were registered later in life

(CNN)It sounds like the plot of a mystery novel.

A controversial one-child policy that resulted in as many as 60 million "missing girls" in China, the most populous country on Earth.

But in a new study, researchers suggest that around 25 million of these girls aren't actually missing, but went unreported at birth -- only appearing on government censuses at a later stage in their lives.

"Most people are using a demographic explanation to say that abortion or infanticide are the reasons (these girls) don't show up in the census and that they don't exist," said John Kennedy, study co-author and political science professor at Kansas University.

"But we find there is a political explanation."

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The 'non-existent' one

A family in rural northwest China

When China implemented the one-child policy in 1979, the government expected local family planning officials to enforce it. However, implementing the rule proved harder in villages, where officials were also members of the community.

Kennedy -- who spent long research stints in rural China -- discovered that in many cases, village officials turned a blind eye to children born outside the one-child policy. They'd let them go unreported in order to maintain good relations with the villagers.

Kennedy said that by the mid-1980s, the Chinese government relaxed one-child policy rules in rural areas, allowing villagers to have a second child if the first was a girl. Yet in the 1990s, Kennedy discovered that lax policy enforcement had allowed families in rural areas to bypass the policy.

A farmer Kennedy spoke with shed light on the situation when he introduced his elder daughter and son by name, but referred to his middle daughter as the "non-existent one."

"He told us that his first daughter was registered but that when his second child, a daughter, was born they did not register her and instead waited to have another child. The third child was a boy; they registered him as the "second" child," said Kennedy.

Looking at official government censuses

Girls born during China's one-child policy sit in a village northwest China.

To supplement the observations they'd collected from interviews with villagers in rural China, Kennedy and study co-author Shi Yaojiang, an economics professor at Shaanxi Normal University, analyzed Chinese population data that spanned a 25-year period.

They discovered that though families didn't register girls immediately after birth or in the months following, they tended to get reported between the ages of 10 to 20.

When the researchers compared the number of children born in 1990 with the number of Chinese men and women in 2010, they discovered four million more people. Of those, there were roughly one million more women than men.

"Between 1990 to 2000, we observed a much later registration for girls. This is as girls might tend to be registered before marriage whereas young boys will get registration earlier for education," said Kennedy.

Kennedy hopes that findings from his study suggest that China's "marriage squeeze" -- where young men can't get married owing to a lack of women -- might not be as serious as previously thought.

In 1958, China introduced 'hukou' or household registration laws that offered state welfare benefits to those connected to it by birth registration. Those whose births aren't registered can't benefit from this system to this day.

"They can't get a job or feel like a part of society," said Kennedy, who hopes that his study will encourage the Chinese government to help register those who remain unregistered to this day.